1. Field of the Invention
The invention relates to an exhaust gas purifying device and method for an internal combustion engine, and more particularly, to an exhaust gas purifying device and an exhaust gas purifying method which are capable of conducting recovery from sulfur poisoning and the like even if extremely low-load operation is continued.
2. Description of Related Art
In an internal combustion engine installed in an automobile or the like, especially in a diesel engine or a lean-bum gasoline engine in which a mixture containing an excessive amount of oxygen (a mixture exhibiting a so-called lean air-fuel ratio) can burn, the advent of an technology for purifying nitrogen oxides (NOx) contained in exhaust gas in the internal combustion engine has been expected.
A technology of disposing an NOx absorbent in an exhaust system of an internal combustion engine has been proposed to meet such a demand. An occlusion/reduction-type NOx catalyst has been known as one type of the NOx absorbent. The occlusion/reduction-type NOx catalyst absorbs nitrogen oxides (NOx) contained in exhaust gas when the exhaust gas flowing into the catalyst exhibits a high oxygen concentration, and discharges the absorbed nitrogen oxides (NOx) and reduces them to nitrogen (N2) when the exhaust gas flowing into the catalyst exhibits a decreased oxygen concentration in the presence of a reducing agent.
In the case where the occlusion/reduction-type NOx catalyst is disposed in the exhaust system of the internal combustion engine, nitrogen oxides (NOx) contained in exhaust gas are absorbed by the occlusion/reduction-type NOx catalyst when the exhaust gas exhibits a high air-fuel ratio during lean-bum operation of the internal combustion engine, and the nitrogen oxides (NOx) absorbed by the occlusion/reduction-type NOx catalyst are discharged and reduced to nitrogen (N2) when the exhaust gas flowing into the occlusion/reduction-type NOx catalyst exhibits a reduced air-fuel ratio.
It is to be noted herein that sulfur oxides (SOx), which are produced through combustion of sulfur contained in fuel, are also absorbed by the occlusion/reduction-type NOx catalyst according to the same mechanism as in the case of NOx. Sulfur oxides (SOx) are not discharged when nitrogen oxides (NOx) are usually discharged and reduced. Thus, if a predetermined amount or more of sulfur oxides (SOx) is accumulated, the NOx catalyst becomes saturated and unable to absorb NOx. This phenomenon, which is referred to as sulfur poisoning (SOx poisoning), causes a decrease in the NOx purification ratio. For this reason, a poisoning recovery process for recovering the NOx catalyst from SOx poisoning needs to be performed at a suitable timing. This poisoning recovery process is performed by allowing exhaust gas with a decreased oxygen concentration to flow through the NOx catalyst while the NOx catalyst is held at a high temperature (e.g., 600 to 650° C.).
However, exhaust gas is below the aforementioned temperature during lean-bum operation. Thus, when the engine is in a normal operational state, it is difficult to raise the bed temperature of the NOx catalyst to a temperature required for the recovery from sulfur poisoning. In such a case, it is possible to decrease the oxygen concentration of exhaust gas while raising the temperature of the aforementioned catalyst by adding fuel to an exhaust passage.
As a method for raising the temperature of the NOx catalyst, an exhaust gas purifying device for an internal combustion engine has been proposed in Japanese Patent Publication No. 2845056. The exhaust gas purifying device for the internal combustion engine disclosed in this publication determines the addition amount of a reducing agent in consideration of the amount of the reducing agent consumed through a reaction with oxygen contained in exhaust gas in an occlusion/reduction-type NOx catalyst and the amount of the reducing agent required for the reduction of nitrogen oxides (NOx) absorbed by the occlusion/reduction-type NOx catalyst. This exhaust gas purifying device thus prevents the reducing agent from being supplied excessively or insufficiently and aims at inhibiting exhaust emission properties from being deteriorated by the discharge of the reducing agent or nitrogen oxides (NOx) into the atmosphere.
On the other hand, in a diesel engine, it is important to remove particulate matters (hereinafter, referred to as “PMs” unless otherwise mentioned) such as soot as a suspended particulate matter contained in exhaust gas. A technology of providing a particulate filter (hereinafter, simply referred to as “filter”) for collecting PMs in an exhaust system of the diesel engine in order to prevent the PMs from being discharged into the atmosphere is therefore well-known in the art. This filter collects the PMs contained in exhaust gas and thus prevents them from being discharged into the atmosphere. However, if the PMs collected by the filter are accumulated on the filter, the filter may be clogged with the PMs. Such clogging raises the pressure of the exhaust gas upstream of the filter, thereby possibly causing reduced output power of the internal combustion engine and damage of the filter. In such a case, it is possible to remove the PMs by igniting and burning the PMs accumulated on the filter. Such removal of the PMs accumulated on the filter is called regeneration of the filter.
In order to ignite and burn the PMs collected by the filter, the temperature of the filter must be raised to a high temperature of, e.g., 500° C. or more. However, since the exhaust gas temperature of the diesel engine is lower than this temperature, it is difficult to remove the PMs through combustion in a normal operation state.
It is possible to use an electric heater, burner or the like to heat the filter to a temperature that causes ignition and combustion of the collected PMs. However, this requires a great amount of energy to be supplied from the outside. Regarding this problem, Japanese Patent Laid-Open Publication No. 6-159037 and the like use a filter carrying a NOx catalyst and a device for supplying hydrocarbons to exhaust gas as a reducing agent. This facilitates combustion of the PMs by using the heat generated by combustion of the hydrocarbons supplied to the exhaust gas in the NOx catalyst.
The aforementioned recovery from sulfur poisoning is carried out with the oxygen concentration of exhaust gas decreased. However, if the reducing agent is added during high-load operation of the internal combustion engine, the reducing agent bums in the occlusion/reduction-type NOx catalyst. As a result, the temperature of the occlusion/reduction-type NOx catalyst rises excessively. This may cause thermal degradation of the occlusion/reduction-type NOx catalyst. Accordingly, it is preferable that the recovery from sulfur poisoning be carried out while the internal combustion engine is in a low-load range.
However, in the case where the internal combustion engine is in an extremely low-load operation state for a long time, for example, in the case where a vehicle having the internal combustion engine is parked in an idle state for a long time or runs in heavy traffic in a town, the internal combustion engine discharges a small amount of exhaust gas and thus the absolute amount of heat generated by the exhaust gas is not enough to raise the overall temperature of the filter (e.g., filter with a 2-liter capacity) carrying the NOx catalyst.
Even if control for regeneration of the PMs accumulated on the filter or control for regeneration of the NOx catalyst from sulfur poisoning (also referred to as control for recovery from sulfur poisoning) need be carried out in such a state, it is impossible to raise the temperature of the NOx catalyst to a temperature range required for such controls. It is therefore impossible to carry out these controls. As a result, the PMs and NOx are not removed, thereby possibly causing insufficient purification of the exhaust gas.